Molecular mixing in Rayleigh-Taylor instability. Part I: Global mixing

This paper describes experiments on the mixing produced by Rayleigh-Taylor instability between two miscible fluids. A layer of brine is placed over a layer of fresh water in a gravitational field, and the ensuing flow is visualized by laser-induced fluorescence and measurements are made of the concentration fields as the flow develops. It is found that largescale disturbances develop which produce intermingling of the fluids, but molecular mixing occurs as a result of small-scale instabilities which grow on the edges of the larger scale motions. When the system overturns stably stratified fluid is produced, and the mixing efficiency of the process is measured and found to be large compared with other forms of mixing. It is suggested that this increased efficiency is due to the fact that much of the mixing occurs when the system is unstably stratified.